DE69020821T2 - Driving method for a liquid crystal display unit. - Google Patents

Description

1. Field of the invention

The present invention relates to a method for driving a liquid crystal display and, more particularly, to a method for driving, in a flicker-free manner, a liquid crystal display using liquid crystal pixels arranged in a matrix.

2. Description of the state of the art

As is known, a liquid crystal display (LCD) has advantages such as low power consumption and easy portability. Therefore, LCDs are widely used in portable calculators and wristwatches to display characters. With the development of office automation, i.e., automation of business machines, LCDs with high performance are required to realize highly integrated business machines. To meet this requirement, a thin film transistor liquid crystal display (TFTLCD) using thin film transistors (TFTS) as switching elements of pixels has been developed and manufactured.

Figure 1 shows a conventional TFTLCD. The TFTLCD comprises pixels P11 to Pnm arranged in a matrix. The pixels are connected to signal lines X1 to Xm and scanning lines Y1 to Yn. A Signal electrode driving circuit 1 and a scanning electrode driving circuit 2 turn on the pixels Pnm and provide a display signal to the pixel.

Figure 2 is an equivalent circuit of one of the pixels of the TFTLCD. The circuit comprises a 3nm liquid crystal pixel and a 4nm switching element, i.e. the TFT. This TFT is usually made of amorphous silicon, polysilicon, a silicon wafer, etc.

To drive the TFTLCD of Figures 1 and 2, the scanning electrode drive circuit 2 provides a scanning pulse to the liquid crystal pixel 3nm through the scanning line Yn. According to a display pattern, the signal electrode drive circuit 1 provides a signal voltage through the signal line Xm. The pulse through the scanning line Yn turns on the TFT 4nm and the signal voltage charges a capacitor 5nm. After the TFT 4nm is turned off, the capacitor 4nm holds the charged voltage until the TFT 4nm is turned on again. The voltage held in the capacitor 5nm is applied to the liquid crystal pixel 3nm to display a dot.

Figure 3 is an equivalent circuit diagram of the TFTLCD in Figure 1. In Figure 3, the TFTLCD includes signal lines X1 to Xm; scanning lines Y1 to Yn; TFTs 411 to 4nm arranged at intersections of the signal and scanning lines; capacitors 511 to 5nm respectively connected to the TFTS; liquid crystal dots 311 to 3nm respectively connected to the TFTS; and a common potential 6 to which one ends of the capacitors and the liquid crystal points are connected.

Referring to Figures 4a to 4c, an operation of the TFTLCD of Figure 3 is described below.

The signal electrode drive circuit 1 applies a voltage signal Vsm having a time/voltage characteristic of Figure 4a to the signal line X (X1, ..., Xm). The scanning electrode drive circuit 2 applies a gate voltage Vgn of Figure 4b to the scanning line Y (Y1, ..., Yn). As a result, a drain voltage VD of Figure 4c for a selected field is applied to a liquid crystal dot located at an intersection of the lines X and Y. At this time, an "ON current" Io is expressed as follows:

Io = Cox u(W / L)(VD - VsN)

{Vgn - Vth - (VD + Vsm)/2} ...(1)

With:

Cox = gate insulation film capacitance;

u = mobility;

Vth = threshold voltage;

W = TFT channel width; and

L = channel length.

As can be seen from equation (1), when the voltage Vsm is positive, the "ON current" is insufficient, so that a waveform of the drive voltage VD may be asymmetrical on positive and negative sides, as shown in Figure 4c. This may cause flickering or flickering.

Each 3nm liquid crystal dot responds to an effective value of the drive voltage, which varies for each field over a voltage level Vcom. Consequently, the transmission, i.e. the intensity of each liquid crystal dot differs for each field, causing the flicker.

When the gate voltage Vgn is turned off, as can be seen from Figure 2, the voltage VD leaks into the liquid crystal dot via a parasitic capacitance Cgd between the gate and the drain and drops by ΔVp which is expressed as follows:

ΔVP = Cgd Vg/Cds + Cs + CLc + Cgd + Cpd ... (2)

with Cds = capacitance between signal line and drain;

Cs = storage capacity;

Cgd = capacitance between gate and drain;

Cpd = capacitance between adjacent signal line and liquid crystal point;

This voltage change ΔVp appears for each field to cause the flicker.

In addition to the two factors mentioned above, there is another factor that causes flicker, i.e. an "OFF current" of the TFT. The "OFF current" changes in response to a gate/source voltage Vgs of the TFT to produce a difference (ΔV+ off - Δ-off) between the positive and negative sides of the pixel voltage VD, thereby causing flicker.

Therefore, there are the following three factors that cause flickering:

(1) insufficient TFT “ON current”;

(2) a leakage of a gate voltage due to a gate/drain capacitance of the TFT; and

(3) a TFT "OFF" current.

As mentioned above, due to the insufficient characteristics of the switching element (TFT), an effective voltage applied to each pixel differs depending on the positivity or negativity of a driving voltage, so that a plane flicker of 30 Hz may occur when a normal field inversion operation is carried out.

To reduce plane flicker, a method of driving a liquid crystal display has been proposed in which the polarity of a driving voltage is inverted within an image.

This technique converts the layer flicker into line flicker or into very small layer flicker, such as pixel flicker, thereby reducing the visible flicker.

Figures 5a to 5c show conventional flicker-free driving techniques disclosed in Japanese Patent Laid-Open No. 60-156095 in which the polarity of a signal line is inverted, in Japanese Patent Laid-Open No. 60-3698 in which the polarities of signal and scanning lines are inverted, and in Japanese Patent Laid-Open No. 60-151615 in which the polarities are inverted for each scanning. Reference is also made to WO-A-87/05141.

Figure 5a shows the field inversion technique, where polarities are inverted for each field.

Figure 5b shows the scan inversion technique, where polarities are inverted for each scan. The inversion is performed not only for each image (frame), but also within an image, thus driving each pixel alternately.

Figure 5c shows the column inversion technique, in which the polarities of signal lines (Figure 3) are alternately inverted. Similar to the row inversion technique, the polarities between frames are inverted to convert plane flicker into column flicker.

It has been experimentally confirmed that the image inversion technique, for example that of Figures 5b and 5c, can theoretically and practically reduce the plane flicker of each image to less than a visible level by equalizing the intensity of each image.

However, the conventional techniques shown in Figures 5a to 5c produce visible horizontal and vertical stripes. This is explained below.

The driving technique of Figure 5a inverts polarities field by field, so the technique is not effective for reducing plane flicker.

The driving method of Figure 5b inverts polarities for each touch, so that the technique is effective for reducing plane flicker, but produces visible horizontal stripes corresponding to the touch lines. In particular, when a motion snapshot by moving a camera, i.e., a so-called PAN, is displayed on a screen and when the eyes of a viewer follow the movement on the screen, the horizontal stripes are particularly visible. A speed of the eyes in a vertical direction on the screen is expressed as follows:

Ve = (2n - 1)ly / Tf

with ly = vertical pixel spacing;

n = 0, 1, 2, ...;

Tf = field period.

When the speed of the eyes coincides with a movement of a horizontal stripe caused by the inversion operation in a frame, the horizontal stripe is seen as if it is stopped. As a result, the horizontal stripe is clearly seen on the screen. This is not advantageous.

The driving method of Figure 5c inverts the polarity of each signal line, so that the technique is suitable for reducing plane flicker, but produces visible vertical stripes. The reason for this is that a color signal G is the most noticeable among color signals R, G and B. Therefore, as shown in Figure 5c, a vertical stripe of color G is formed. Similar to the case of Figure 5b, the vertical stripe is particularly visible when a viewer's eyes move horizontally to follow a movement on the screen.

The conditions that make the vertical and horizontal stripes visible are considered below.

Figures 6a and 6b show experimental results of visibility/discrimination threshold characteristics with respect to a moving line. As can be seen from the figures, high-speed motion produces a spatial frequency characteristic with a low bandpass and low-speed motion produces a bandpass characteristic with a maximum Sensitivity at 3 cycles/degree. The maximum sensitivity of a slightly moving motion is higher than that of a stopped motion. In each case, a contrast and a spatial frequency determine a visible area and the conventional flicker-free driving techniques, which operate on the existing TFT characteristics, produce visible vertical and horizontal stripes.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method for driving a liquid crystal display that can provide high-quality images without flickering and with reduced vertical and horizontal stripes by sensing liquid crystal pixels line-sequentially. This object is achieved by a method according to claim 1.

According to a first aspect of the present invention, each display pixel comprises a liquid crystal dot, a switching element and a color filter to which a color signal R, G or B is supplied. A plurality of the pixels are arranged in a matrix to form a liquid crystal display. The display pixels arranged in rows and columns are connected to a plurality of signal lines and scanning lines which are orthogonal to each other. When scanning the display pixels line-sequentially, the polarities of the signal voltage are inverted for each scanning. In addition, when scanning the signal lines to which the color signals R, G and B are supplied provided, phases of inverted polarities are shifted.

According to a second aspect of the present invention, each display pixel comprises a liquid crystal dot, a switching element and a color filter to which a color signal of R, G or B is supplied. The color filters for the R, G and B signals in one row are shifted by 1/2 pitch from those in an adjacent row. A plurality of the pixels are arranged in a matrix. The display pixels arranged in rows and columns are connected to a plurality of signal lines and scanning lines which cross orthogonally with each other, thereby forming a liquid crystal display. In line-sequentially scanning the display pixels, the phase and cycle of polarity inversion are changed for each signal line to which the color signal of R, G or B is supplied.

As described above, according to the first aspect of the present invention, polarities of signal lines are inverted for each scan in line-sequential scanning of display pixels. Assuming that the transmittances of the display pixels R, G and B for positive and negative polarities are R+, G+, B+, R⁻, G⁻, B⁻, the intensities I⁺ and I⁻ are expressed as follows:

I&spplus; = 0.59G&spplus; + 0.3R&spplus; + 0.11B&spplus;

I- = %59G⊃min; + 0.3R- + 0.11B⊃min;

When driving phases of display pixels R, G and B are shifted, an amount FR of flicker is expressed as follows

When the phases of the display pixels G and B are shifted, the flicker amounts FG and FB are expressed as follows:

If G+ = R+ = B+ = T+, G⊃min; = R⊃min; = B⊃min; = T⊃min; and T⊃min; = T⊃min;+T, the following results:

From the above, ΔT-F with T+ = 1 is obtained as shown in Figure 8. From this figure it can be seen that an effective driving method consists in reversing the polarity of one of the color signals R, G and B from that of the other two.

The second aspect of the present invention inverts polarities of signal lines for each scan. In addition, the second aspect arranges each group of three color filters R, G and B in a delta and changes the phases of polarity inversion of color signals at the color filters for respective signal lines. As a result, a delta-by-delta intensity change can occur in an image. This is a so-called delta inversion driving method. According to this method, vertical stripes are embedded so as not to be visible.

These and other objects, features and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings show:

Figure 1 is a circuit diagram schematically showing a conventional TFTLCD;

Figure 2 is an equivalent circuit diagram showing a pixel of the TFTLCD of Figure 1;

Figure 3 is an equivalent circuit diagram of the TFTLCD from Figure 1;

Figures 4a to 4c are waveforms showing drive and pixel voltages according to a conventional LCD drive method;

Figures 5a to 5c are explanatory diagrams showing conventional LCD driving methods;

Figures 6a and 6b show visibility discrimination threshold characteristics illustrating the visibility of vertical and horizontal stripes;

Figure 7 is a plan view showing the essential part of an LCD driven by a driving method according to a first embodiment of the present invention;

Figure 8 is a characteristic diagram showing a relationship of a transmission difference to a flicker amount in an alternating drive operation and an effect of the first embodiment of the present invention;

Figure 9 is an explanatory diagram showing the LCD driving method according to the first embodiment of the present invention;

Figure 10 is a view showing a relationship between the number of horizontal pixels and the spatial frequencies of horizontal and vertical stripes shows, for explaining an LCD driving method according to a second embodiment of the present invention;

Figures 11a to 11c are views showing vertical and horizontal stripes occurring in respective driving methods;

Figures 12a to 12c are views showing the LCD driving method according to the second embodiment of the present invention; and

Figures 13a and 13b are views showing waveforms of signals applied to pixels via signal lines according to the embodiment of Figures 12a to 12c.

DETAILED DESCRIPTION OF THE EMBODIMENTS

A liquid crystal display (LCD) according to the embodiment of the present invention will be described below with reference to the drawings.

In Figure 7, the LCD comprises signal lines X1 to Xm, scanning lines Y1 to Yn, thin film transistors (TFTs) 411 to 4nm connected to intersections of the signal and scanning lines, capacitors 511 to 5nm connected to the TFTs, respectively, liquid crystal dots 311 to 3nm connected to the TFTs, respectively, color filters G, R and B arranged for the liquid crystal dots, and a common potential 6 to which one ends of the Liquid crystal dots 311 to 3nm and capacitors 511 to 5nm.

A signal electrode drive circuit 1 applies signal voltage pulses to the TFTLCD via the signal lines X1 to Xm, and a scanning electrode drive circuit 2 applies scanning signal pulses to the TFTs 411 to 4nm via the scanning lines Y1 to Yn. Flickering occurs due to the positive and negative changing polarity of a signal voltage applied to each liquid crystal dot.

Assuming that the transmission of the color pixels R, G and B for positive and negative polarities are R+, G+, B+, R-, G- and B-, then intensities I+ and I- are expressed as follows:

I&spplus; = 0.59G&spplus; + 0.3R&spplus; + 0.118&spplus;

I- = %59G⊃min; + %3R- + %11B⊃min;

Here, an amount F of flicker is defined as follows:

In normal field inversion operation, the value F is defined as follows:

Assuming that G⁻> G⁺, R⁻> R⁺ and B⁻> B⁺, it follows from the above equation that the flicker occurs strongly because the transmission of each color pixel changes in phase.

To reduce the flicker, the phases of the color signal voltages R, G and B can be shifted to drive them from G+, W- and B+ to G-, R+ and B- (only R is inverted) as shown in Figure 9. Amounts of flicker at this time are expressed as follows:

Here we assume that G+ = R+ = B+ = T+, G⊃min; = R⊃min; = B⊃min; = T⊃min; and T⊃min; = T⊃min;+ΔT. Then we get the following:

From the above, ΔT-F with T+ = 1.0 is found as shown in Figure 8. From this figure, it can be seen that changing the polarity of only one color signal of the R, G and B color signals from that of the remaining two is effective. However, this is only effective for displaying a white color. For a monochromatic display, the flicker is reduced.

If the R, G and B signals in a field are inverted at the same phase, flickering may occur, but no vertical and horizontal stripes may appear in the image. However, if the phases are shifted as mentioned above, colors in the frame may change, but the vertical and horizontal stripes may not be visible.

The above embodiment arranges each group of three color filters into a delta. It is also possible to arrange the color filters into a mosaic.

Next, the second embodiment of the present invention will be explained.

As mentioned above, the conventional flicker-free LCD driving techniques produce vertical and horizontal stripes in a frame. The visibility of these stripes is closely related to their spatial frequencies. This will be examined below. When examining the vertical and horizontal stripes on a display screen, the stripes are examined from a position at a distance "3H" from the screen, three times the height "H" of the screen. From the line inversion driving method, the following results:

Assuming that NV = 488, it follows that NLN = 12.8 [C/d], with:

Nv = the number of vertical lines;

NLN = spatial frequency of horizontal stripes.

The following results for the column inversion control method:

with NH = the number of horizontal pixels;

NSN = spatial frequency of vertical stripes.

From equations (3-1) and (3-2), a relationship of the number of pixels to the spatial frequencies of vertical and horizontal stripes is obtained, which is shown in Figure 10.

Since human eyes are most sensitive to green (G), vertical and horizontal stripes are observed at the distances shown in Figure 10 depending on the driving methods. This fact has been confirmed experimentally.

Compared with the scanning power inversion drive method of Figure 11a, the column inversion drive method of Figure 11b produces a greater degree of visible vertical stripes with a large pitch. This is because every other G pixel is inverted to form a redundant pitch. To deal with this, a half pitch inversion method shown in Figure 11c can reduce the visibility of the vertical stripes and produces a greater degree of visible vertical stripes with a large pitch compared with the Line inversion control method high quality images.

The method of Figure 11c is realized in the manner shown in Figure 12a. In Figure 12a, color filters G, R and B are arranged in a Δ (delta) shape with a shift of 1/2 pitch between adjacent lines. Since the color filters R, G and B are arranged in a delta shape with inverted polarities, this method is called a delta inversion driving method.

A spatial frequency NDN of vertical stripes in the delta inversion driving method is expressed as follows:

NDN = 3/4 NH tan 1º [c / d] = 2NSN

Since a pixel pitch Ly of the vertical stripes is narrow, and in addition, the vertical stripes are nested, they are not visible. Furthermore, as can be seen from Figure 10, with an increase in a horizontal resolution and the number of effective horizontal pixels, the spatial frequencies of the vertical stripes increase, so that the vertical stripes become more invisible. In recent years, the horizontal resolution and the number of horizontal pixels have been increased, so that the present invention will be even more useful.

The delta inversion control method with color filters arranged in a delta can be used depending on a The method of connecting signal lines can be realized in two ways as shown in Figs. 12b and 12c. In Fig. 12b, different color pixels are connected to the same signal line, so that the color pixels can be classified depending on their signal lines into those whose polarities are changed for each scanning line and those whose polarities are changed for each field. In the latter color pixels, there are some whose phases differ from the others by 180 degrees. Accordingly, there are three kinds of driving states in one frame. The driving waveforms of the method of Fig. 12b are shown in Fig. 13a.

In Figure 12c, a signal line is connected to the same type of color pixels. In this case, the phase of one color signal among three color signals must be shifted by 180 degrees from that of the remaining two while inverting their polarities for each scanning line. Drive waveforms of the method of Figure 12c are shown in Figure 13b.

In summary, the present invention can reduce flickering and make vertical stripes invisible, thereby providing high-quality images on an LCD. In addition, the present invention can narrow pitches of vertical and horizontal stripes occurring in a frame to make them invisible and reduce flickering.

Various modifications will be possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof as defined in the appended claims.

Claims (6)

1. A method for driving a liquid crystal display with a plurality of display pixels (P11-Pnm) arranged in a matrix and a plurality of signal (X1-Xm) and scanning (Y1-Yn) lines which cross orthogonally and are connected to a respective display pixel (P11-Pnm), each of the pixels (P11-Pnm) comprising a liquid crystal dot (311-3nm), a switching element (TFS, 411-4nm) and a color filter (R, G, B) to which one of the three color signals (R, G, B) is supplied, and the three color filters (R, G, B) are arranged one after the other along each scanning line (Y1-Yn), the method comprising the following steps: a) inverting polarities of each signal lines (X1-Xm) applied to the three color filters (R, G, B) for each scan; and b) inverting the phase of the polarity inversion of one of the three color filters (R, G, B) with respect to the remaining two on each scanning line (Y1-Yn) for each scan. 2. Method according to claim 1, characterized by application of the method to the liquid crystal display, wherein the three color filters (R, G, B) are arranged in a vertical strip arrangement. 3. Method according to claim 1, characterized by application of the method to the liquid crystal display, characterized in that the three color filters (R, G, B) are arranged in a mosaic-like arrangement. 4. A method according to any preceding claim, characterized by matching (Figure 9) the phase of the polarity inversion of each color filter between each adjacent scanning lines (Y1-Yn). 5. Method according to claim 1, characterized by inverting (Figures 11c, 12) the phase of the polarity inversion of each color filter between each adjacent scanning lines (Y1-Yn) 6. Method according to claim 5, characterized by an application of the method to the liquid crystal display, characterized in that the color filters (R, G, B) are offset by 1/2 pitches in adjacent scanning lines (Y1, Y2; Figures 5, 9) to form a delta arrangement with three color filters (R, G, B) each.